1. Field of the Invention
The present invention relates to the field of integrated circuits, and more particularly to temperature stabilization of circuits providing some form of reference to other circuits.
2. Prior Art
In many systems, it is required to generate one or more references which maintain predetermined accurate values, system to system and over the desired operating temperature range of the system. In many applications, it is desired to generate the reference on an integrated circuit for use by other circuits on the same chip. In other applications, it is desired to generate the reference on an integrated circuit for use by other integrated circuits, or alternatively, for use by other integrated circuits and also for use by other circuits on the same chip as the circuit generating the reference. Such references include, but are not necessarily limited to, voltage references, current references and frequency references.
The preferred embodiment of the present invention described herein relates specifically to voltage references. In general, it is difficult to make high precision voltage references since these require a tight tolerance on the reference voltage and a low drift with temperature. Normally, to achieve better precision, the room temperature reference voltage is trimmed at wafer sort. This can be easily done to .+-.0.1% of the nominal desired voltage. But after packaging, the reference voltage can shift. This shift is a function of process, layout and packaging (and probably some other factors). To achieve accuracy better than around .+-.0.5%, one usually needs some form of post-package trim. This adjusts the reference voltage to the nominal value at room temperature; however, it doesn't guarantee low temperature drift.
One very commonly used voltage reference is the bandgap reference. This type of reference combines two components of voltage, one of a positive temperature coefficient and one of a negative temperature coefficient, to achieve a combined voltage reasonably temperature insensitive. In particular, the base-emitter voltage of a junction transistor is given by the following equation: ##EQU1## where: T=temperature
T.sub.0 =an arbitrary reference or starting temperature PA1 I.sub.C =the transistor collector current PA1 I.sub.C0 =collector current for which V.sub.BE0 was determined PA1 V.sub.go =semiconductor bandgap voltage extrapolated to a temperature of absolute zero PA1 V.sub.BE0 =base to emitter voltage V at T.sub.0 and I.sub.C0 PA1 q=electron charge PA1 n=structure factor PA1 K=Boltzmann's constant
The dominant terms are the first two terms: ##EQU2## and since V.sub.go is larger than V.sub.BE0, the net result is a negative temperature coefficient.
If one subtracts the VBEs of two identical transistors Q.sub.1 and Q.sub.2 operating with different collector currents, there results: ##EQU3##
This usually is expressed in terms of current densities J.sub.1 and J.sub.2 in the two transistors as follows: ##EQU4##
A bandgap reference takes advantage of these two components by adding a V.sub.BE (or a forward biased diode voltage drop) to a properly weighted V.sub.BE difference of two transistors operating at different current densities.
The temperature drift of bandgap references is a strong function of the reference voltage. Minimal temperature drift usually occurs around a bandgap voltage of 1.23 V. A 1% change in this reference voltage can easily shift the temperature drift by 30 ppm/.degree.C. Even if the reference voltage is trimmed to the ideal value, there is variability due to process, device matching, circuit design, and (possibly) packaging effects.
The result is that it's difficult to make precision references (&lt;.+-.0.1% initial accuracy, &lt;.+-.10 ppm/.degree.C.) without very careful trimming, etc. Even if the tolerance is tight and average drift is low, references tend to have nonlinear temperature drift (curvature) that constitutes a major error.